Name

ST_Area — Returns the area of the surface if it is a Polygon or
MultiPolygon. For geometry, a 2D Cartesian area is determined with units specified by the SRID. For geography, area is determined on a curved surface with units in square meters.

Synopsis

float ST_Area(geometry g1);

float ST_Area(geography geog, boolean use_spheroid=true);

Description

Returns the area of the geometry if it is a Polygon or
MultiPolygon. Return the area measurement of an ST_Surface or
ST_MultiSurface value. For geometry, a 2D Cartesian area is determined with units specified by the SRID. For geography, by default area is determined on a spheroid with units in square meters.
To measure around the faster but less accurate sphere, use ST_Area(geog,false).

Enhanced: 2.0.0 - support for 2D polyhedral surfaces was introduced.

Enhanced: 2.2.0 - measurement on spheroid performed with GeographicLib for improved accuracy and robustness. Requires Proj >= 4.9.0 to take advantage of the new feature.

Return area square feet and transform to Massachusetts state plane meters (EPSG:26986) to get square meters.
Note this is in square feet because 2249 is
Massachusetts State Plane Feet and transformed area is in square meters since EPSG:26986 is state plane Massachusetts meters

Return area square feet and square meters using geography data type. Note that we transform to our geometry to geography
(before you can do that make sure your geometry is in WGS 84 long lat 4326). Geography always measures in meters.
This is just for demonstration to compare. Normally your table will be stored in geography data type already.